Papermaking comprises taking a slurry of papermaking raw materials at a consistency (weight percent solids) in the range 0.1 to 1.0 weight percent and dewatering it to form a sheet with a final consistency of about 95 weight percent. Paper machines accomplish this dewatering through a series of different processes which include from the beginning to end: 1) gravity or inertial dewatering (early forming section of the machine); 2) vacuum dewatering (late forming section of the machine); 3) press dewatering (press section of the machine); and 4) thermally evaporating the water (dryer section of the machine). The cost of dewatering increases in going from 1 to 4, which makes it advantageous to remove as much water as possible in the earlier stages. The rate of paper production or, equivalently, the machine speed is dictated by the rate at which the water can be removed, and consequently, any chemical treatment which can increase the rate of water removal has value for the papermaker. Many grades of paper require the use of retention aid chemicals for their manufacture in order to retain the fine particles found in the raw materials used to make the paper. It is well known in the paper industry that these retention aids can also enhance the rate of gravity, inertial, and vacuum dewatering or drainage, as it is often called. Such retention chemicals include the well known flocculants, coagulants, and microparticles used in the industry. Existing laboratory free and vacuum drainage tests can readily identify the drainage effects of these retention aid chemicals.
The production rate for the vast majority of paper machines is limited by the drying capacity of the machine's dryer section. Consequently, the consistency of the paper sheet leaving the press section and going into the dryer section is most often critical in determining the paper machine speed or production rate. The effects of chemical additives on press dewatering are unclear with little information available on this topic. The effect of retention aid chemicals on press dewatering is often reported to be detrimental as a consequence of the decreased consistency entering the press as a result of increased water retention or reduction in press efficiency resulting from a loss in sheet formation. Both these factors arise from the flocculation of the papermaking particles by the retention chemicals. Because the consistency of the sheet leaving the press section is most often the most critical factor in determining machine speed, any treatment capable of increasing this consistency would obviously be highly desirable. Currently, no chemical treatments are known to be marketed as commercial press dewatering aids, although anecdotal reports suggest that some polymers can favorably effect out going press consistency. Accordingly, there is an ongoing need to develop compositions having effective press dewatering activity.
Glyoxylated polyvinylamides prepared from glyoxal and polyvinylamide in a mole ratio of 0.1 to 0.2 are disclosed as wet strength resins in U.S. Pat. No. 3,556,932.
Low molecular weight glyoxylated cationic polyacrylamides prepared from glyoxal and cationic polyvinylamide in a ratio of 0.1-0.5:1 are disclosed as temporary wet strength resins in U.S. Pat. No. 4,605,702.
A method of imparting strength to paper by adding to a pulp slurry a mixed resin comprising an aminopolyamide-epichlorohydrin resin and a glyoxylated acrylamide-dimethyl diallyl ammonium chloride resin prepared from glyoxal and acrylamide-dimethyl diallyl ammonium chloride copolymer in a molar ratio of about 2-0.5:1 is disclosed in U.S. Pat. No. 5,674,362.